[SYCL] Fix macro which is used to build debug proxy loader (#21245)
UR_WIN_PROXY_LOADER_DEBUG_POSTFIX macro in ur_win_proxy_loader.cpp
allows to control whether to load ur_loaderd.dll or ur_loader.dll. Wrong
macro name was used to build proxy loader, so ur_win_proxy_loaderd.dll
used ur_loader.dll mistakenly.